Re: [Detnet] Magnus Westerlund's Discuss on draft-ietf-detnet-mpls-11: (with DISCUSS)

[Magnus Westerlund <[e-mail]>]

On Wed, 2020-09-09 at 20:34 +0000, Janos Farkas wrote:
> Hi Magnus,
> 
> I think the parameter you are looking for is there in section  4.2.2.3. Packet
> Ordering Function Processing (
> https://tools.ietf.org/id/draft-ietf-detnet-mpls-11.html#pof-requirements),
> just encoded a different way; in particular in the number of packets:
> “Note that an implementation MAY wish to constrain the maximum number of out
> of order packets that can be processed, on platform-wide or per flow basis.
> Some implementations MAY support the provisioning of this number on either a
> platform-wide or per flow basis. The number of out of order packets that can
> be processed also impacts the latency of a flow.”
> 
> Ultimately, the latency requirements of flows have to be met. The maximum
> packet size of a DetNet flow is known, see item c) MaxPayloadSize in 
> https://tools.ietf.org/html/draft-ietf-detnet-flow-information-model-10#section-5.5
>  and max-payload-size in 
> https://tools.ietf.org/html/draft-ietf-detnet-yang-07. 
> So, maximum POF buffer size to be applied for a flow can be determined; can be
> converted to time and taken into account during latency engineering of a
> DetNet network; thus bounded latency can be provided.

So given that the buffer is specified in either bytes or simply packets to be
buffered will result in that the POF buffering time becomes packet flow
dependent and not bounded in time. So if you make the calculation for a DETNET
flow thinking it will send 500 packets per second evenly spaced. Then the a
buffer of 5 packets would represent an upper limit 1/100th of second. If then
the flow sends only 100 packet per second then suddendly the 5-packet buffering
would represent 1/20th of a second. Thus defining it in packets or size doesn't
work, the upper buffering time needs to be defined in time to provide a bounded
latency. 

> 
> I fully agree with you that this needs to be taken into account. Together with
> what the set of documents that specify DetNet provide, I think it is fine what
> we have in the MPLS data plane draft. When it comes to configuration - as you
> pointed out - I think the job we need to do is to pay attention to do it right
> in the YANG draft, which is still under development.

From my perspective I think this document needs to correctly document the
requirement to bound in time the POF and having a configuration. The actual
detail of the configuration is fine to leave to another document. 

Cheers

Magnus Westerlund 


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